The friction coefficient of composite packaging film refers to the ratio of the friction between the composite film or between the composite film and the surface of other objects to the vertical force acting on its surface. It is a unitless value, and its size can reflect between the films Under the same conditions as the object, the amount of friction can reflect the smoothness of the composite film. It is the inherent physical properties of the composite film under certain processing or environmental conditions, and is only related to the material composition of the inner and outer surface film and the surface roughness of the film. The friction coefficient should be adapted to the subsequent processing and use of the composite film, especially the friction coefficient of the heat seal layer of the composite film. Its stability is one of the important indicators to be controlled during the production of the composite film. Many composite film manufacturers are very confused about the friction coefficient being stable and meeting standards. Even after exploring for years, they still cannot find an effective solution.
I. Standard of friction coefficient of composite film substrate
Composite film coefficient of friction with the main surface of the non-corona-treated surface of the film about the inner and outer, the relevant national standards friction coefficient non-corona-treated surface of the base film part, such as:
GB / T 20218-2006 «biaxially stretched polyamide (nylon) film » specified BOPA≤0.6.
GB / T 16958-2008 «biaxially oriented polyester film for packaging» BOPET specified static friction coefficient ≦ 0.65; dynamic friction coefficient ≤0.55.
GB / T 10003-2008 «general purpose biaxially oriented polypropylene (BOPP) film» not make a specific predetermined value in the coefficient of friction BOPP.
GB / T 4456-2008 «packaging, polyethylene blown film >> without making the relevant provisions of recommendation is determined by the supply and demand sides negotiated.
27740-2011 «cast polypropylene (CPP) film» specified CPP≤0.5 GB / T.
From the above values of the specific standard, the basic upper limit value is a predetermined coefficient of friction, in actual use, the user often lower limit value of the coefficient of friction is also required. In the course of packaging film, both friction and resistant force, friction as both a positive role, but also counterproductive, if the friction coefficient is too small, the same is not conducive to the use of a composite membrane, which is why some of this standard does not define data s reason.
Currently used web to continuously improve the speed of automatic packaging equipment, the friction coefficient of the composite membrane of claim increasingly stringent, high speed packaging machines, composite packaging film coefficient of friction generally 0.05 standard error indicators, such as, customer acceptance criteria of the friction coefficient 0.3, measured coefficient of friction between 0.25 and 0.35 to be acceptable, beyond this range, considered unacceptable. Moreover, some packaging machines require a high coefficient of friction packaging material , it requires special processes, targeted production, the friction coefficient can not apply the rigid standards.
2 ﹑ test mode coefficient of friction
Friction occurs between two objects and is divided into dynamic friction and static friction. The determination of friction coefficient is generally calculated by the amount of friction between objects. The physical definition of the coefficient of friction, in addition to press the GB / T 10006-1988 «plastic film and sheet friction coefficient measurement method >> predetermined, we can list several ways to test the coefficient of friction:
1. Feeling test
In order to verify the smoothness between the heat-sealing layers of the composite film or the surface of the composite film, the feel test is the easiest and most commonly used method at the production site. Fold the composite film directly in half. The thumb and forefinger or middle finger press the two films back and forth with appropriate force. If the sliding is smooth and the resistance is small, it can indicate that the friction coefficient of the film is small, otherwise it means that the friction coefficient is large. Each person's feel is different, and there are many subjective factors. Therefore, this method can only qualitatively explain the size of the friction coefficient. The friction coefficient of the composite film cannot be quantified, and the dynamic friction coefficient can only be roughly grasped. The static friction coefficient cannot be determined. Experienced flexible packaging practitioners, using this feel to determine the coefficient of friction of a composite film is very practical, and can quickly determine the adaptability of the film, including the impact on the use of subsequent processes.
In the past, there was a simple and practical test tool for measuring the coefficient of friction by using the slope adjustment of the slope and sliding the slider down. The flat composite film sample was perfectly attached to the slope of the test tool. The slope that can be adjusted manually or automatically Put the slider at the end where the slope can be lifted slowly, and slowly lift the slope. When the slider slides, immediately stop the slope lifting. Write down the tilt angle α of the slope at this time. The decomposition of gravity G, the force of the vertical slope is: N1 = G * COSα; the force of the parallel slope is N2 = G * SINα. According to the definition of the friction coefficient, the friction coefficient u = N2 / N1 = G * SINα / G * COSα = tgα, The friction coefficient value can be determined based on the tangent trigonometric function value of the inclination angle α of the inclined plane.
The friction coefficient measured in this way is often a static friction coefficient. The dynamic friction coefficient is estimated based on different materials and different temperature conditions. Generally, the static friction coefficient is subtracted from 0.01 to 0.05 as the dynamic friction coefficient. This test method requires less instrument investment and simple operation, but the accuracy is very poor. The flatness of the slider and the slope itself affects the data. Different operators have different judgments, so the error is particularly large.
3.Rally machine test
The universal tensile machine used in the flexible packaging industry can be used as a tool for testing the friction coefficient of composite films. The tension machine is equipped with a smooth plate with a fixed pulley at one end and a G = 200g slider. The sample of the composite film to be tested is tightly adhered to the smooth plate and the film surface is kept flat. The fiber surface of the slider or The film surface of the composite film is in contact with the sample film on the smooth plate. The slider is placed on a horizontal plate. The thin traction rope that pulls the slider must be parallel to the plate through the fixed pulley. The traction rope passes the fixed pulley and is connected to the tensile machine vertically. Set the appropriate traction speed at the stretching end of the machine, such as 100mm / min. When the tensile machine starts to pull the slider, the moment when the slider starts to slide, the tensile force peaks. This peak is the static friction force F1 (unit: gf). Then the slider moves forward at a constant speed. During this process, the relatively stable pulling force value F2 (unit: gf) displayed on the tensile machine is recorded. In this way, according to the definition of the friction coefficient, the composite film can be calculated on the slider fiber surface or steel surface or film. The coefficient of surface friction, static friction coefficient is: ustatic = F1 / G, and the dynamic friction coefficient is: udynamic = F2 / G. The friction coefficient tested by this method is relatively accurate, and the value has a certain degree of accuracy and is relatively reliable.
4.Test of friction coefficient tester
The friction coefficient tester is an instrument developed for testing the dynamic and static friction coefficients of materials. The ease of operation and data accuracy have been greatly improved. For example, the common Guangzhou standard GM-4 friction coefficient tester can measure the dynamic and static friction coefficients of composite films. Perform accurate tests. This instrument has matching control software, and can print the movement curve of the slider online with the computer. Equipped with a standard slider, the measurement accuracy can reach 0.001, and the test can be achieved in the temperature range of 5 ~ 50 ℃, and the LCD displays the test results. After the instrument sets the test parameters, simply place the sample to be tested flat on the test bench of the instrument, place the slider on the test position, press the instrument's automatic calibration button, and then press the test button. The instrument starts and the slider slides a certain distance It will automatically stop, and the static friction coefficient and dynamic friction coefficient of the sample will be displayed at the display position of the instrument. At present, professional friction coefficient testers not only have high accuracy and simple operation, but also can automatically display test results, automatically save each test data for easy retrieval, and print test reports online. Many composite film processing companies have introduced professional Coefficient of friction tester.
By testing the friction coefficient of the composite film, you can use this as a basis, select the type and content of the opening agent and slip agent to be added, and you can control and adjust the smoothness of the film material to meet the requirements for use.
3. The influence of friction coefficient on the use of composite film
The coefficient of friction is an index to measure the sliding characteristics of composite materials. The smoothness of the surface of a composite film affects the friction. Friction is very important for the use of composite films. Packaging materials for different uses have different requirements for the coefficient of friction. The friction force during use is both dynamic and resistance. Fluctuation of friction coefficient will bring about changes in friction force, so the friction coefficient must be stabilized in a certain range according to requirements to ensure the smooth use of the composite film.
1. The friction coefficient of the composite film is too small, and the static friction between the inner surface layer and the outer surface layer of the composite film will be very small when winding up. In this way, when the number of rolled-up rice gradually increases, the winding diameter will become larger and larger, and the friction cannot support. If the film is rolled up neatly, the end surface of the film will protrude and sag on the other side, forming a common "channeling", which is very unfavorable for subsequent processing or use. For example, in the slitting process, the width of the cut film is small, for example, the width of the finished film is less than 60mm. When the roll diameter is larger, the unloading from the machine should be flat or carried, which will cause loose coils. Processing is cumbersome, not only time consuming and laborious, but also a lot of material wasted. In order to increase friction and prevent uneven winding or unwinding, only the winding tension is increased, but excessive tension will cause other quality problems.
2. The composite film coil for automatic packaging generally requires a small friction coefficient in the inner layer and a moderate friction coefficient in the outer layer. The friction coefficient of the inner layer is too small, which may cause the stacking material to be unstable during bag making and cause pattern misalignment. Too much friction coefficient of the outer layer will cause the resistance in the packaging process to cause the material to stretch and deform. Too small will cause slippage during dragging, electric eye tracking and misplacement of the bag.
3. When making the bag, if the friction coefficient of the composite film is too small, the friction between the film and the equipment guide roller will be very small, so that the composite film will slide on the machine, causing the tracking of the electric eye to be misaligned, the deviation of the bag making pattern, and the bag. The length and length are uneven, and the proportion of defective products greatly increases, and even production is impossible. If the friction coefficient is too large, the friction between the film and the machine parts is very large, which makes it difficult for the machine to drag the material, that is, the common film cannot be pulled on the machine. It will severely break the film, and it can not produce normally.
4. The friction coefficient is too large, which makes it difficult to open the bag, especially the large-capacity bag type packaged by hand. Although the opening of the bag is not good, it does not entirely depend on the smoothness of the heat-sealing layer of the composite film. However, if the heat-slip of the film is not good and the outer surface is slippery, it is difficult to open the bag by hand, which greatly affects Packaging efficiency. The coefficient of friction on the surface of the bag is too small, and it will be very slippery between bags. It is very difficult to bind the bags. Sometimes the bags that are tied together will easily spread and need to be reorganized, which will make the bagmaker very distressed.
Therefore, the friction coefficient of the inner surface layer and the outer surface layer of the composite film should be selected appropriately, so as to ensure the stable use of the composite film.
Factors affecting the coefficient of friction
As for the friction coefficient of the composite film, there are many influencing factors, mainly the properties of the inner and outer surface substrate film. During processing, external factors change the properties of the inner and outer surface substrate film. First, the composition changes, or the surface state changes. The friction coefficient of the composite film is not only related to the material composition of the inner and outer surface film, but also related to the slipping agent and the opening agent used for processing the substrate film. In general, the friction coefficient of the composite film is not stable with the addition of the slipping agent and the opening agent. Type, addition amount, migration direction and migration amount are related. The role of the slip agent is to add another layer of lubricant to the surface of the plastic film to reduce the surface friction coefficient to the required value. The effective ingredients of the slip agent such as erucic acid are usually organic. The role of the opener is to change the surface of the film from the original extremely smooth state to a "concavo-convex" uneven state. When the two films are close to each other, the contact tightness is eliminated. The active ingredients of the opener such as silica powder are usually Inorganic substances, these two additives work on different principles. However, while the opening agent prevents the film from adhering, it also reduces the friction coefficient. The factors that cause the change of slip agent and opener in the composite film are usually the factors that affect the friction coefficient, such as:
1. Influence of additives on friction coefficient
Adding additives (slip agents, opening agents) in the process of making plastic films is a common way to adjust the surface friction coefficient of plastic films. Slip agents are divided into inner slip agents and outer slip agents: the inner slip agent promotes the relative movement of polymer macromolecular chains or segments, thereby improving the mobility of the material's macromolecules. The outer slip agent is in phase with the polymer group. Poorly polar organic chemicals whose molecules move to the film surface to form oily lubricants can increase the smoothness of the film surface and reduce the coefficient of friction on the film surface. The opening agent is mainly a solid powder with a small particle diameter. It can form "concave and convex" when it is added to the surface layer of the film, so that the area of the film layer is reduced when it comes into contact with the layer, and the resistance is reduced when it slides relatively. The addition amount of the opening agent and slip agent of the inner layer material, and the stiffness and smoothness of the film will affect the friction coefficient of the composite film.
2. Influence of the material structure on the membrane surface
Different types of plastic films have different material structures, and some have smooth surfaces that slide on each other. The effective contact surface is large and the sliding is difficult. Such a film has a large coefficient of friction. Some film surfaces are rough, and there are some gaps between the film surfaces when they are close to each other. It is easier to slide each other. Such a film has a low surface friction coefficient. So different films need to be selectively added with additives.
3.Effect of high and low temperature use on friction coefficient
The temperature conditions of the film during use may be different. Above 30 ° C, the friction coefficient will gradually increase, because the commonly used lubricant is close to its melting point and becomes sticky, which greatly increases the friction coefficient. For example, the curing temperature of the composite film is too high. The slip agent melts and loses its effect at high temperatures. The RCPP used for high-temperature cooking is aged at high temperatures for a long time, which easily causes the friction coefficient to increase. When used under low temperature conditions, the precipitation of additives affects the coefficient of friction. For example, the slipping agent of PE film is precipitated severely at low temperatures, and the coefficient of friction will therefore become smaller.
4. Glue penetrates into the inner surface of the composite membrane too much
If the heat seal layer is thin or corona treatment is excessive, the amount of glue penetration will increase, which will also affect the friction coefficient. Some data show that after the composite process, the coefficient of friction of the substrate film will increase by about 0.1 ~ 0.2, because the adhesive absorbs the slip agent, and during the production process, each guide roller of the compound machine will stick out a part of the slip agent.
5.Too much foreign matter is attached to the composite film
Foreign matter includes moisture and dust condensed on the composite film when the humidity in the space is high. If the humidity is high, the moisture will condense on the composite film too much, and more often it will penetrate inward from the end of the roll film, which will increase the film. Coefficient of friction. In the case of too much dust, the sticky dust will increase the friction coefficient, and the dust like slipper or opening agent will reduce the friction coefficient.
5. How to keep the friction coefficient stable
Because there are many factors affecting the friction coefficient of the composite film, it is necessary to consider it from multiple angles to maintain its stability. For the smooth use of the composite film, in addition to controlling the friction coefficient of the film itself, it must have stable temperature and humidity conditions during use. At the same time, the surface conditions of the equipment parts in contact with the film cannot be ignored.
1. According to the actual situation, determine the coefficient of friction coefficient, select the appropriate slip agent and opening agent, and keep the raw material ratio of the substrate film stable, and standardize the production process conditions.
2. Allow standard cooling or retention time before use to allow the slip agent in the composite film to function. For example, the PE film cannot be compounded immediately after it is produced. At this time, the slip agent is not completely precipitated, and at the same time, some of the slip agent will migrate to the glue layer after compounding. If the storage time is too long, the proportion of slip agent is large. Excessive precipitation also affects the friction coefficient, which is disadvantageous for subsequent use.
3. According to the characteristics of the slip agent, determine the temperature conditions of the composite film processing, and stabilize the temperature and humidity conditions of the space in which the composite film is used. For example, the characteristics of organic slip agents, when the temperature decreases, it will migrate to the surface of the film, causing the friction coefficient of the film surface to decrease; when the temperature increases, it will migrate to the interior of the film, causing the friction coefficient of the film surface to increase.
4. In order to reduce the serious influence of temperature on the precipitation and migration of organic slip agents and keep the friction coefficient stable, an appropriate amount of inorganic opening agent can be added to the raw material formula.
5. According to the actual required friction coefficient, strictly control the curing conditions, including curing time and temperature. Avoid affecting the precipitation and migration of additives due to instability of curing conditions.
6. Select glues that are stable in reaction with additives. According to actual experience, determine the good mix of glue type and surface film additive ratio. Because different glues have different degrees of adsorption of additives, for example, after the PE film of the same formula and different glues are compounded, the friction coefficient of the PE film will be different.
7. For composite films used under high temperature conditions, high temperature resistant slip agents should be added, or the amount of organic slip agents should be appropriately increased to reduce the serious influence of temperature on the friction coefficient.
Remedial measures for friction coefficient runaway
Sometimes the same kind of composite film is suddenly too slippery or too astringent. This problem is caused by a variety of reasons. If this kind of friction coefficient is out of control, the production company needs to think of remedies to avoid losses. Uncontrolled friction coefficient will affect the normal operation of the membrane.
If the coefficient of friction is too large, the following remedies are commonly used:
1. Dusting: The film is dusted on a special duster. The solid fine powder plays a role of easy opening and smoothness on the surface of the film. This is the most commonly used method to reduce the friction coefficient of composite films. The dusting facility can be used alone or on a rewinding machine, inspection machine, extruder or bag making machine. According to the required friction coefficient, the appropriate amount of powder can be selected.
2. Select a certain low-temperature environment and leave it for a period of time to allow the slip agent to be analyzed before use.
3. If the coefficient of friction of the single-layer substrate film is too large, it cannot be used, and it can be recycled and redone.
4. Reduce the roughness of the guide roller of the bag making machine or packaging machine, reduce the friction between the film and the guide roller as much as possible, and make up for the defect of large friction coefficient.
5. Rewind the roll film with small tension to reduce the bonding force between the film and the film, and reduce the effect of large frictional resistance.
If the coefficient of friction is too small, the following methods can be used:
1. Place in a curing room at a certain temperature to allow the slip agent to migrate to the glue layer, reduce the amount of slip agent in the surface layer of the film, and it can also be used at slightly higher temperature conditions. The slip agent will lose some effects.
2, if the coefficient of friction is caused by too much slip agent deposited is reduced, several times rewinding tension may be greater, so that the inner surface of the slip agent is transferred to the outer surface or a portion of guide rollers, to reduce the surface coefficient of friction.
3, the outer skin layer is one side of the composite film PET or OPP, if the friction coefficient is low, a corona treatment may be performed in playing without affecting the surface layer. Was treated with a configuration of PET // AL // NY // composite films of PE, PET layer after corona treatment, the friction coefficient of the outer surface of PET is about 0.05 increased.
4, increase the humidity of the environment, environmental humidity increases agglomeration adverse effects on the composite film of water will increase the friction, slippery film reduction caused.
5, increasing the roughness of the piece of equipment in contact with the membrane, increase the friction, reduce the friction caused synovium and small misalignment phenomenon.
6, when using the appropriate unwinding tension increases, increasing resistance, and synovial avoid dislocation.
The friction coefficient depends on the composite film material of the membrane surface, and the opening and the role of slip agents and the like additives, additives for reducing various unfavorable factors play a role, it is possible to obtain stable coefficient of friction. More measures to control stable coefficient of friction need to constantly sum up and accumulate in the production practice, so that the friction coefficient of the composite film production and use will always play a beneficial role.